Studies by the applicant have demonstrated the existence of a neural circuit in birds by which retinal illumination may regulate choroidal blood flow in the eye. Light-elicited increases in choroidal blood flow have been demonstrated in mammals. The proposed studies will more fully elucidate the anatomical organization and function of the neural innervation of the choroid, particularly with respect to parasympathetic innervation of the choroid (which may be involved in reflexive increases in choroidal blood flow). Histochemical (largely immunohistochemical) techniques will be used to describe the source, distribution and target structures of parasympathetic fibers, sympathetic fibers and sensory fibers (each of which can be identified by the transmitters or peptides they contain) innervating the choroid in pigeons and rabbits. Pathway tracing techniques (cobalt labeling, horseradish peroxidase and autoradiography) will be used to elucidate the central pathways involved in parasympathetic control of the choroid, starting with the central parasympathetic preganglionic neurons and progressing in a retrograde direction. Particular effort will be devoted to identifying central visual pathways that might control choroidal blood flow. Because of the similarities between human and monkey retina, the distribution of the different types of nerve fibers innervating the monkey choroid will also be determined. Another set of studies will focus on the functions of the already demonstrated visual-parasympathetic pathway to the choroid in birds. The role of this circuit in the control of choroidal blood flow will be evaluated, using lesions or microstimulation of this circuit and ocular tissue temperature or hydrogen washout desaturation to measure choroidal blood flow. Finally, the effects of lesions of this circuit on photoreceptor viability will be investigated in birds housed under normal conditions and in birds housed under conditions known to be deleterious to photoreceptors. Since the choroid plays a vital role in the sustenance of the retina, neural control of the choroid may have an important influence on retinal function. Disruption of this neural control may result in abnormal or nonadaptive regulation of blood flow and lead to heightened susceptibility of retinal photoreceptors to the deleterious effects of light, possibly leading to photoreceptor degeneration.